73 



Part II. Growth and Yield Studies 



The objects of the survey were twofold. The one inchided locating, 

 mapping, and clas-sifying the forests; the other was a study of the pro- 

 ductiveness of different soils in terms of forest crops. 



Growth studies were made upon individual trees, and upon plots. 

 The studies on individual trees were made with the object of determining 

 the average rate of growth in height, diameter, and cubic contents for a 

 given species upon a given soil type. When a comparison is made of all 

 species growing upon a given soil type these average growth rates show 

 which are the fastest growing trees for that soil type. See Tables 1 and 

 2, pp. 78-80. When a comparison is made of the rates of growth of a 

 single species on different types of soil, there is shown the soil type best 

 fitted for that species. See Table 3, pp. 81-89. The studies on plots 

 were made to determine the number of trees and volumes per acre pro- 

 ducd on a fully stocked stand for virgin plots and for even-aged plots 

 at ten year intervals. 



(1) Studies of Growth Rates of Individual Trees 

 The chief factors which influence the rate of growth of a tree are 

 (1) atmospheric, including temperature, light, humidity, and precipita- 

 tion as the most important; (2) soil, including water contents, gas con- 

 tents, soil composition, soil temperature, exposure, slope, character of 

 the surface and altitude; (3) biotic, or plants and animals which react 

 upon forest vegetation. It is impossible to secure exact duplication of 

 these dozen or more factors even in trees growing upon the same acre, 

 hence there results a variation in the rate of growth of individual trees 

 quite independent of the variation due to qualities inherent in different 

 species. In the effort to standardize as far as possible those factors which 

 influence the rate of growth of a given species, all measurements were 

 made on plantation or forest-grown trees ; the soil type as identified by 

 the State Soil Survey was used as a basis for soil standardization ; the 

 measurements were worked up for trees growing in even-aged stands 

 and all-aged stands separately ; and as many felled trees as possible were 

 measured. Average, rather than abnormally rapid or slow growing trees, 

 were measured. No division is made between data collected in different 

 parts of the state, other than those derived from even or uneven aged 

 stands and from the soil type. 



Under the soil survey made by the University of Illinois Agricul- 

 tural Experiment Station, the soils have been classified as unglaciated 

 or as belonging to a definite period of glaciation ; and as bottomland and 

 swamp, or upland timber and prairie ; and some 150 different types have 

 been identified in the 93 counties surveyed. A list of those counties for 

 which information is available is given on page 83. This information 

 gives, for any definite area, the soil types represented and a description 

 of the physical and chemical characters for each type. 



The studies of growth rates are based upon this system of soil classi- 

 fication and are carried on separately for bottomland and upland soils 



